Projection of dynamic picture content onto a screen, or as a virtual picture, using a digital, reflective liquid-crystal-based image generator is based, in accordance with conventional technology, on projection devices having an imaging optical channel, or three channels, whose optical paths unite upstream from the optical projection element in order to realize the color mixture.
In particular, US 2009 323 028 A1 shows picoprojectors that are LED-illuminated in a color-sequential manner. In addition, US 2009 237 616 A1 describes a projection display comprising three color channels combined upstream from the optical projection element.
However, if one reduces the dimensions of the systems known in conventional technology in order to realize miniaturized picoprojectors, brightness losses of the projected picture will result, in particular. Miniaturization of known projection systems is possible only to a limited extent due to the transmissible luminous flux being limited by the small surface area of the image generator existing in said systems. This connection is determined by the basic optical law of étendue maintenance. The étendue, or light conductance, of a light sourceE=4πn2A sin Θresults from its luminous surface area A, the half-angle of divergence Θ, and the relative refractive index n, and remains constant in the case of ideal optical imaging. Real optical elements increase the étendue and/or decrease the system transmission. Thus, a minimum object surface area may be used for a source having a given level of luminance for a minimally transmissible luminous flux within a projecting optical system.
A general problem is that due to optical laws (e.g. natural vignetting, abberations), for single-channel projection systems the design length of the system increases to the same extent as this area that is to be imaged increases, which makes miniaturization more difficult.